Adrian: The example you linked is worth exploring in detail:
1. The Law of Non-Contradiction says nothing can be both A and not-A.
2. Only men have penises. (By definition)
3. A hermaphrodite has a penis.
4. Therefore, a hermaphrodite is a man.
5. Only women have vaginas. (By definition)
6. A hermaphrodite has a vagina.
7. Therefore, a hermaphrodite is a woman.
8. A woman is not a man. (By definition)
9. Therefore, a hermaphrodite is not a man.
10. Therefore, a hermaphrodite is both a man and not a man.
11. Therefore, a hermaphrodite is a counter-example to the Law of Non-Contradiction.
12. Therefore, the Law of Non-Contradiction is not true.
For paradoxes such as this, there is either something wrong with the rule or something wrong with the definitions.
The easiest way to refute this is to look at the definitions (steps 2 and 5). For instance, step 2 says: "only men have penises," which would equate to "for anything x, if x has a penis, then x is a man." Yet step 3 directly contradicts step 2 ("A hermaphrodite has a penis"), since we know, by definition, that a "hermaphrodite" has the sexual characteristics of both a man and a woman (assuming,
arguendo, that true hermaphrodites actually exist). Thus there is something wrong with the definition in step 2.
The conclusion in step 12, then, is only true if we disregard the fact that it is based on flawed premises; i.e. there is an internal contradiction between the definitions in steps 2 and 5 and the fact statements in steps 3 and 6.
A simple tweaking of the definitions can solve the paradox. A revised step 2 would then read: "for anything x, if x has only male sex organs, then x is a man." In that event, the hermaphrodite would be
neither a man nor a woman: that doesn't violate the law of non-contradiction, but it does give us a case of a
tertium quid which contradicts the law of the excluded middle ("something is either A or not-A").